1. Field of the Invention
This invention relates to systems which absorb noise or sound or influence the acoustical resonance behaviour of rooms and chambers. More particularly it relates to active systems to absorb noise or to influence room acoustics.
2. Prior Art
In some applications in the field of acoustics devices are needed which reflect or absorb acoustical waves in a specified way. Often these devices should not reflect any acoustical waves.
At high frequencies this specified behaviour, e.g. no reflection, can be achieved by simple, passive constructive means, i.e. the of use absorptive materials like foam rubber or glass wool, and by giving the non-reflecting surface a special shape. However at low frequencies the dimensions of absorptive structures get large and impractical.
Some passive resonant structures are known under the name xe2x80x9csound trapsxe2x80x9d (U.S. Pat. Nos. 2,502,020, 2,706,530, 4,319,661, 4,548,292, 5,210,383). These devices use passive resonators (e.g. Helmholtz-Resonators) which increase the ability to absorb sound of low frequency. However the dimensions are still in relation to the efficiency. The resonance frequencies are mainly defined by the dimensions of the resonance chambers. For low frequencies these dimensions must be large again.
Another related invention is Max Hobelsberger""s device for simulation of an acoustical impedance (U.S. Pat. No. 5,812,686) which is used to establish a specified acoustical impedance. This device is an active device which uses an electroacoustic transducer which acts, together with a control system, as acoustical impedance.
It is an object of this invention to provide relatively small, active acoustical elements which can be used as building elements for the creation of sound absorbing systems.
The invented system follows the function principle, that the air pressure in a chamber inside a housing is influenced by an electroacoustic transducer together with control system in a predetermined manner which simulates certain chamber characteristics, e.g. a certain chamber volume.
The system comprises a housing with a front wall and an electroacoustic transducer arranged inside the housing. The front wall allows a pneumatic communication between inside and outside of the housing. For example it may be equipped with one or more openings which permit air to flow between inside and outside. Or it may be equipped with displaceable membranes which are moved by pressure differences between inside and outside the housing. The housing could be either of the closed type, or it could be a vented housing.
The inner transducer is built into an opening of an inner wall of the housing. This inner wall separates an air chamber which adjoins the holes in the outer wall from one or more other chambers within the housing. The inner transducer is preferably an electrodynamic transducer with membrane, similar to an electrodynamic loudspeaker, however other types of transducers may be used too. In case of an electrodynamic transducer its membrane is driven by a coil which is placed in the magnetic field of the transducer""s magnet system. The inner transducer influences with the movement of its membrane the pressure inside the chamber which adjoins the front wall which is equipped with the holes. Pressure sensing means, e.g. a pressure sensor, is mounted inside this chamber to measure the air pressure which is influenced by the inner transducer and by the air pressure outside of the housing, i.e. in front of the front wall. The output signal of the pressure sensing means is conveyed to calculating means, i.e. a micro-processor or analog circuits comprising operation amplifiers, which produce further signals. These signals are applied as setpoint values of movement to a controller (microprocessor or analog circuit) which controls via a power amplifier the movement, e.g. the speed, of the inner transducer""s membrane.
The controller forces the membrane of the inner transducer to move with momentary values of movement, e.g. with a speed, according to the setpoint values of movement. These setpoint values are calculated, based on the pressure values, in such a way that the desired acoustical properties are achieved.
For a fuller understanding of the nature of the invention, reference should be made to the following detailed description of the preferred embodiments of the invention, considered together with the accompanying drawings.